Check for the obvious culprits: no keyframes on any of your meshing attributes? What about things like particle radius?

Something else that might be related--and I'm not 100% sure about this--is that there might be some outlying particles that are throwing off the meshing resolution. I believe that the mesh gets calculated differently depending on the size of the bounding box of the particle object. So if one particle is sneaking off and traveling far away from the group--or if a faraway particle suddenly dies for some reason, and the bounding box shrinks--you might see a big change in the mesh like this.

As I say, I'm not sure, but it might be worth looking into. Good luck.

I managed to solve this one, it seemed to be a problem with the output mesh settings. The Max Triangle Resolution was set too low so that at a certain point in the timeline the mesh grew beyond the resolution of the voxel grid and so it seemed to be tessellated with less detail from that point onwards.

Increasing the Max Triangle Resolution allowed for the mesh to grow further at a good resolution.